The invention relates generally to gas turbine engines, and, more specifically, to micro-channel cooling therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the gases in a high pressure turbine (HPT), which powers the compressor, and in a low pressure turbine (LPT), which powers a fan in a turbofan aircraft engine application, or powers an external shaft for marine and industrial applications.
Engine efficiency increases with temperature of combustion gases. However, the combustion gases heat the various components along their flowpath, which in turn requires cooling thereof to achieve a long engine lifetime. Typically, the hot gas path components are cooled by bleeding air from the compressor. This cooling process reduces engine efficiency, as the bled air is not used in the combustion process.
Gas turbine engine cooling art is mature and includes numerous patents for various aspects of cooling circuits and features in the various hot gas path components. For example, the combustor includes radially outer and inner liners, which require cooling during operation. Turbine nozzles include hollow vanes supported between outer and inner bands, which also require cooling. Turbine rotor blades are hollow and typically include cooling circuits therein, with the blades being surrounded by turbine shrouds, which also require cooling. The hot combustion gases are discharged through an exhaust which may also be lined, and suitably cooled.
In all of these exemplary gas turbine engine components, thin metal walls of high strength superalloy metals are typically used for enhanced durability while minimizing the need for cooling thereof. Various cooling circuits and features are tailored for these individual components in their corresponding environments in the engine. For example, a series of internal cooling passages, or serpentines, may be formed in a hot gas path component. A cooling fluid may be provided to the serpentines from a plenum, and the cooling fluid may flow through the passages, cooling the hot gas path component substrate and coatings. However, this cooling strategy typically results in comparatively low heat transfer rates and non-uniform component temperature profiles.
Micro-channel cooling has the potential to significantly reduce cooling requirements by placing the cooling as close as possible to the heated region, thus reducing the temperature difference between the hot side and cold side of the main load bearing substrate material for a given heat transfer rate. Current techniques for forming micro-channel cooled components typically require the formation of access holes for the micro-channels using line-of-sight processing. In addition, current techniques for forming access holes through the top opening of a micro-channel are typically suitable for drilling an access hole with an effective exit diameter (based on the area enclosed) equal to or less than that of the top opening size of the channel. That is, for current machining techniques, one dimension of the tool must typically be less than the opening width, and this sets an upper limit on the size of access hole the tool can machine in that same dimension or direction.
Moreover, conventional machining methods break up the formation of multiply featured requirements into distinct and separate operations, often using differing machine tools. For example, micro-channel cooling passages may be made by milling the channels, followed by down-hole drilling of the access holes, then followed by shaping of the channel exits. Typically a different tool head would be used in each operation, which would involve re-positioning the tool or part, and would also create some transition or discontinuity in the resulting channels and holes. For flow passages such as micro-cooling of turbine parts, these discontinuities and start-stops are undesirable, leading to material flaws and dimensional changes.
It would therefore be desirable to provide improved methods for machining cooling channels and their associated access holes and channel exits. It would further be desirable to provide methods for forming a larger sized access hole through an existing restricted entry surface.